1. Field of the Invention
The present invention relates to a backlight unit having a light guide plate as a flat light source and a liquid crystal display device using the backlight unit.
2. Discussion of the Background
Liquid crystal display devices have been widely used as display devices for OA equipment such as personal computers or word processor and audiovisual equipment such as TV sets, video movies and car navigations because the display devices have many advantages such as lightweight, compact, and low power consumption. In these liquid crystal display devices, a backlight unit having a light guide plate is equipped at a rear side of the liquid crystal display device to illuminate the liquid crystal display panel from the rear side. Recently, a flat light source formed of light emitting diodes (LEDs) has been used. Here, the backlight unit is classified into an edge light type and a direct backlight type. For example, the edge light type includes a light source such as discrete light sources (LEDs) and fluorescent tubes to illuminate light to an edge of a light guide plate facing the liquid crystal display panel. On the other hand, in the direct backlight type, a plurality of light sources made of linear tubes such as the fluorescent tubes are arranged at a back side of the liquid crystal display panel interposing a light diffusion plate therebetween. The edge light type is superior to the direct backlight type in the aspect of compactness. Therefore, the edge light type is suitable to the displays such as mobile electronic equipments or note type personal computers.
In the edge light type, it is necessary to hold the light guide plate and the light source by a frame of the liquid crystal display panel. For example, a construction to clip the light guide plate between the frame and a back cover or another construction to hold the light guide plate in the frame by a double face tape is proposed. The former construction may be superior to the latter because an additional element such as the double face tape is not required to attach the light guide plate and the manufacturing process is simplified.
When the light guide plate is built onto a frame, it is necessary to precisely position the light guide plate. For example, Japanese patent application No. 2005-315957 describes a technology in which a first concave groove and a second concave groove are formed in a side edge of the light guide plate. The first concave grove is used to position the light guide plate and the second concave grove is used to fix the light guide plate with the frame by screws. The respective grooves have a pair of side walls which intersect with another side wall arranged therebetween in parallel with the edge of the light guide plate and taking the form of an acute angle to the other side wall to prevent looking of shining from the side walls of the grooves.
Meanwhile, a narrow frame in the liquid crystal display device has been widely adopted recently. To match the narrow frame, when a flexible wiring substrate is retrieved from a substrate equipped with a light source such as light emitting diodes (LEDs), it is required to make a space to pass around the flexible wiring substrate by arranging the shape of the light guide plate. Practically, if a part of the side edge of the light guide plate is cut out in a groove shape to make a space to pass around the flexible wiring substrate, some rectangular corner portions in the light guide plate may be formed, which causes bright lines to be viewed or a concentration of stress being applied to the corner portions. The stress may further cause destruction of the light guide plate in a following mechanical test.